The present invention relates to a toner image fixing device and, more particularly, to the structure of a toner image fixing portion for a laser printer or a copying machine and to a method for controlling the temperature of the heater of the fixing portion.
A toner image fixing device is intended for fixing a toner image onto a sheet of paper by fusion. The heating is done by using a heating means either in contact with a sheet of paper or at a distance therefrom. A fixing heated-roller method is usually adopted.
The fixing heated-roller method is such that a toner image developed with toner on a sheet of paper is fixed by means of heating while the paper passes through a fixing roller which is heated at a specified temperature by a heater that is axially mounted thereon and a paper presser and a feeding roller rotate while in contact with the fixing roller.
In the past, a conventional fixing heater was used such as a halogen lamp that was a silica tube having a tungsten filament mounted therein and filled with an inert gas containing a very small amount of halogen. The lamp produced Joule's heat when an electric current flowed through the tungsten filament having a high melting point.
Japanese laying-open patent No. 57-82961 (Appl. No. 55-158843) describes a conventional halogen lamp for use in copying machines, which is a transparent body made of quartz glass accommodating a tungsten coil filament containing doping elements as a light emitting portion and filled with inert gas and reactive carrier gas.
The referred prior art is concerned with a halogen lamp filled with gas. The present invention relates to a heating roller, which includes a nichrome wire heater in a silica tube with open ends, and to a new method of temperature control for the heating roller.
A conventional halogen lamp-heater which is composed of a silica tube with terminals fitted at the respective ends thereof with a seal of molybdenum foil and with a doubly coiled tungsten filament stretched between the terminals within the silica tube. The sealed silica tube is filled with an inert gas containing a very small amount of halogen. Both terminals have respective lead wires connected to the tungsten filament which is heated by an electric current supplied from an external power source through the lead wires.
The heating portion H of the halogen lamp is a coiled portion of the tungsten filament. However, a region A near each terminal portion may become hot by heat conduction. The molybdenum foil is a sealing material having a coefficient of thermal expansion selected to prevent leakage of the halogen gas from the silica tube but it may be oxidized at temperatures higher than 350.degree. C., resulting in the deterioration of its sealing quality. To avoid this, the sealing portion is provided with a means for protecting against the rise of the temperature therein.
As described previously, the length and position of the heating portion H are determined by the maximum width of a sheet of paper onto which a toner image is to be fixed. When the conventional halogen heating lamp was designed in order for the sealed portion to have a molybdenum foil at a temperature not higher than 350.degree. C., the lamp was necessarily elongated in order to have the elongated non-heating portions A, B and C (e.g., not shorter than 30 mm). As a result, the fixing unit was enlarged and the copying machine had an increased width.
When a heater made of, e.g., nichrome wire, having a large heating capacity which does not require filling with halogen gas nor the use of molybdenum foils, is used in the fixing portion of a laser printer or copy machine working under severe temperature conditions, the conventional temperature control method cannot regulate the temperature within a constant range, i.e., the temperature will vary from the preset range.